Regenerating anion exchange zone containing hexavalent chromium

ABSTRACT

In recovering hexavalent chromium from chrome plated steel strip which is wetted with plating solution as a result of dragout from the plating process, an aqueous solution containing hexavalent chromium in relatively low concentration is concentrated in a process comprising treating in a first cation exchange zone, treating in an anion exchange zone, regenerating the anion exchange zone and treating the effluent of regeneration in a cation exchange zone different from the first cation exchange zone. The concentrated solution is passed to a storage zone and recirculation is carried out between the storage zone and the plating zone and solution is passed from the storage zone to an evaporation zone and more concentrated solution is passed from the evaporation zone to the storage zone. In regenerating the anion exchange zone, only a portion of the hexavalent chromium is removed.

United States Patent 1191 Smith 1 51 May 20, 1975 [75] Inventor: RobertB. Smith, Crown Point, Ind.

[73] Assignee: National Steel Corporation,

Pittsburgh, Pa.

[22] Filed: June 4, 1973 [21] Appl. No.: 366,544

[52] US. Cl. 423/54; 423/6585; 210/30; 210/31; 210/32; 210/37; 210/38;204/51; 75/101 BE [51] Int. Cl ..C01g 37/14; B01d 15/06 [58] Field ofSearch 423/54, 6585; 204/51; 210/37, 38, 3032; 75/101 BE [56] ReferencesCited UNITED STATES PATENTS 2,733,204 1/1956 Costa 423/54 X 3,223,62012/1965 Obuhofer 210/37 X 3,658,470 4/1972 Zievers et a1. 210/37 X3,681,210 8/1972 Zievers et a1. 204/51 X OTHER PUBLICATIONS Paulson etal., Plating, Sept. 1953, pp. 1005-1009. Culotta et al., Plating, Mar.1970, pp. 251-255.

Primary Examinerl-lerbert T. Carter Attorney, Agent, or Firm-Shanley,ONeil and Baker [571 ABSTRACT In recovering hexavalent chromium fromchrome plated steel strip which is wetted with plating solution as aresult of dragout from the plating process, an aqueous solutioncontaining hexavalent chromium in relatively low concentration isconcentrated in a process comprising treating in a first cation exchangeZone, treating in an anion exchange zone, regenerating the anionexchange zone and treating the effluent of regeneration in a cationexchange zone different from the first cation exchange zone. Theconcentrated solution is passed to a storage zone and recirculation iscarried out between the storage zone and the plating Zone and solutionis passed from the storage zone to an evaporation zone and moreconcentrated solution is passed from the evaporation zone to the storagezone. In regenerating the anion exchange zone, only a portion of thehexavalent chromium is removed. I

8 Claims, 2 Drawing Figures REGENERATING ANION EXCHANGE ZONE CONTAININGHEXAVALENT CHROMIUM BACKGROUND OF THE INVENTION This invention relatesto regenerating an anion exchange zone incident to recovering hexavalentchromium and increasing the concentration of the same in aqueoussolution. Such allows practical use for plating of chromium values whichwould otherwise be lost.

Methods have been disclosed for recovery of hexavalent chromiuminvolving washing plated articles wetted with chromium-containingsolution, recovering an aqueous solution containing hexavalent chromiumin relatively high concentration, recovering an aqueous solutioncontaining hexavalent chromium in relatively low concentration,concentrating low concentration solution by ion exhange and regenerationof an anion exchanger, and concentrating by evaporation, In this regard,see Paulson et al, Plating, pages 1000l009, September, I953; Culotta etal, Plating, pages 251-255, March, 1970; Zievers et al, US. Pat. No.3,658,470.

It is an object of this invention to provide a novel method ofregenerating an anion exchange zone in which resin in the hydroxide formhas been used to remove hexavalent chromium in the form of anions from asolution, whereby to maximize the concentration of hexavalent chromiumin the effluent provided by regeneration,

This object and others will be evident from the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B constitute a flowsheetdepicting a process for chrome plating steel strip and for recoveringhexavalent chromium from the solution wetting the plated strip for reuseand illustrate the inventive concepts herein.

DETAILED DESCRIPTION With continuing reference to FIGS. 1A and 1B of thedrawings, steel strip having been preliminarily treated, for example inelectrolytic cleaning and pickling steps, follows a travel path andpasses successively through chrome plating tanks 12, 14, 16 and 18 whichdefine a plating zone. In each of the plating tanks 12, 14 and 16, thestrip follows the travel path so as to enter a plating tank in asubstantially horizontal plane, then passes over a contact roll andadvances downwardly, then passes under a sink roll and advancesupwardly, then passes over another contact roll and leaves the tank in asubstantially horizontal plane. In the plating tank 18, the stripfollows the travel path so as to enter the tank in a substantiallyhorizontal plane, then passes over a contact roll and advancesdownwardly, then passes under a sink roll and advances upwardly, thenpasses over a deflector roll and leaves the tank in a substantiallyhorizontal plane. In FIG. 1A the contact rolls at the strip entrancesides of tanks 12, 14, 16 and 18 are respectively denoted 12a, 14a, 16a,and 1841'; the sink rolls are respectively denoted 12b, 14b, 16b, and18b; the contact rolls at the exit sides of the tanks 12, 14 and 16 arerespectively denoted 12c, 14c, and 16c; and the deflector roll at theexit side of tank 18 is denoted 180. The contact rolls 12a, 14a, 16a,18a, 12c, 14c, and 160 perform a deflecting function and also render thestrip cathodic. At the entrance end of each of the tanks 12,

l4, l6 and 18, snubber rolls respectively denoted 12d, 14d, 16d and 18dforce the strip against adjacent contact rolls to provide good contact.

In the first three tanks, that is tanks 12, 14 and 16, there are twosets of anodic grids which are vertically oriented with one of the setsbeing between the contact roll at the entrance end of a tank and a sinkroll and the other set being between a sink roll and the contact roll atthe exit end of a tank so that the strip passes through one set of gridson its downpass through a tank and through the other set on its up pass.The grids for the downpass are respectively denoted 122, Me and 16e, andthe grids for the up pass are respectively denoted 12f, 14f and 16f. Inthe fourth tank, tank 18, there is only one set of vertically orientedgrids, and this set is between the contact roll at the strip entranceend of the tank and the sink roll 50 that the strip passes through thisset on the downpass and does not pass between any grids on the up pass;this set of grids is denoted 186.

The electrolyte in each of the tanks 12, 14, 16 and 18 is the same,namely an aqueous solution of chromic acid, and is depicted as a bodyofliquid in each tank extending to a level above the anodic grids. Thechemistry of chromic acid is quite complicated and what exactly ispresent is not known. However, at the pH ofless I than 1 which ischaracteristic of the electrolyte utilized,

the chromic acid is mostly in the form of dichromic acid (H Cr O Theconcentration in the electrolyte of the chromic acid expressed as gramsof CrO per liter is in the range of to 250. Small amounts ofsilicofluorides (magnesium, sodium or potassium) and sulfate in the formof sulfuric acid are typical additives.

Prewet sprays 19 are present at the entrance end of tank 12 upstream ofcontact roll 12a to spray plater solution, that is electrolyte, at eachside of strip before it reaches the contact roll, The reason for this isto put a good conducting medium on the strip so that it will make goodcontact with the contact roll thereby negating arcing between thecontact roll and the strip.

The strip having been chrome plated exits from tank 18 and followstravel path 10 through a tank 20. The strip enters the tank 20 in ahorizontal plane, passes over a deflector roll 20a, then turnsdownwardly traveling vertically, then passes under a sink roll 20b, thenadvances vertically upwardly and passes over a deflector roll 20c andthen advances horizontally out of the tank. The tank 20 contains thesame solution as each of the plating tanks 12, 14, 16 and 18 but noelectrolytic action is imparted during the passage of the strip throughthis solution. Passage of the strip through the tank 20 has the effectof removing objectionable oxide coating from the strip.

At the exit side of tank 20 downstream of deflector roll 200 is a pairof wringer rolls 22 between which the strip passes. These rolls 22 wipeplater solution from the strip to minimize dragout of electrolyte fromthe plating process.

The tanks 12, 14, 16, 18 and 20 are continuously supplied withelectrolyte, that is plater solution, respectively through valved lines12g, 14g, 16g, 18g and 20g. These lines are supplied with platingsolution by a header 24. Header 24 also communicates with valved lines26a and 2612 which in turn communicate with sprays 19. The header 24communicates with and is fed by line 28 which in turn communicates withand is fed by valved line 30 which in turn communicates with platersolution storage tank 32 which defines a plater solu- I from solutionwetting the strip.

tion storage zone. A pump 34 is provided in line 28 to continuously movethe plater solution from tank 32 via lines 30 and 28 and into header 24and from there via lines 26a and 26b and 12g, 14g, 16g, 18g and 20g intotanks l2, l4, 16, 18 and 20. A valved line 36 communicates with line 30so that plater solution can be drained from the system. Overflow outletsare provided in each of the tanks 12, 14, 16, 18 and 20, and platersolution continuously overflows through these outlets through therespective lines 12h, 14h, 16h, 18h and 2011. These overflow linescommunicate at their downstream ends into a main exit line 38 whichcommunicates with another line 40 which in turn communicates with theplater storage tank 32. Thus, there is provided between tank 32 andbetween tanks 12, 14, 16, 18 and 20 a recirculation loop wherebyrecirculation is continuously carried out between the storage zonedefined by tank 32 and the plating zone defined by tanks 12, 14, 16 and18.

The plater storage tank 32 is also in a recirculation loop with a heatexchanger 42 defining a heat exchange zone with .a valved line 44communicating between tank 32 and heat exchanger 42 and line 46 leadingback from heat exchanger 42 into storage tank 32. Line 44 contains apump 48 whereby plater solution is recirculated through this loop. Wateris passed through heat exchanger 42 countercurrent to the platersolution as indicated by lines 50a and 50b to remove heat from theplater solution (that is, from the plating solution storage zone definedby tank 32). This is done because there is a heat buildup in the platingportion of the system due to the grid current. In other words, the heatgained because of the current supplied by the grids exceeds the heatlost through the tanks and pipes of the system. Inasmuch as the bestcathode efficiency is achieved utilizing a plater solution having atemperature of 105 to 125F., control of heat buildup is desirable.

The chrome plated strip having had undesirable oxides removed in tank 20and having passed through wringer rolls 22 still is wetted with asignificant amount of plating solution comprising aqueous chromic acidsolution. So as to recover the hexavalent chromium from such solution,the strip is advanced along travel path through a first rinse tank 52and a second rinse tank 54. The strip is then passed through a chemicaltreatment tank 56. The strip is then passed through a third rinse tank58 to recover hexavalent chromium The tank 52 contains a strip entranceend deflector roll 52a, a sink roll 52b and a strip exit end deflectorroll 52c and the strip following travel path 10 turns downwardly overroll 52a and moves vertically downwardly turning under roll 52b and thenadvances vertically upwardly over roll 52c whereupon it leaves tank 52.

The tank 54 contains a strip entrance end deflector roll 54a, a sinkroll 54b and a strip exit end deflector roll 54c. The strip enteringtank 54 after passage out of tank 52 turns downwardly over roll 54a andmoves vertically downwardly, then turns under roll 54b, and thenadvances vertically upwardly and turns over roll 54c and moves out oftank 54.

The tank 56 contains a strip entrance end contact roll 56a, a sink roll56b and a strip exit end contact roll 56c. The strip exiting from tank54 passes over roll 56a and then passes vertically downwardly, thenpasses under roll 56b, then advances vertically upwardly and passes overroll 56c exiting from tank 56 horizontally.

The tank 58 contains a strip entrance end deflector roll 58a, a sinkroll 58b and a strip exit end deflector roll 58c. The strip exiting fromtank 56 passes into tank 58 and over roll 58a, then advances verticallydownwardly, then passes under roll 58b, then passes vertically upwardly,then passes over roll 58c and exits horizontally from tank 58.

In tanks 52 and 54, the strip is subjected to rinsing to wash off platersolution on the strip by dragout from the plater tanks 12, 14, 16 and 18and from tank 20. In the chemical treatment tank 56, the strip issubjected to electrolytic treatment to provide a hexavalent chrome oxidecoating. In tank 58, washing is provided to remove solution on the stripby dragout from the chem treat tank 56.

In the chemical treatment tank, the electrolyte is preferably aqueouschromic acid solution containing 35 grams per liter of chromic acidexpressed as CrO Electrolytic action is supplied in this tank, and itoper ates and is constructed generally as one of the plater tanks 12,14, 16 or 18 with vertically oriented anodic grids 56e and 56f on eitherside of the strip respectively on a downpass and on an up pass andcontact rolls 56a and 56c rendering the strip cathodic and snubber roll56d forcing the strip against contact roll 56a at the inlet to the tank.As with the plater tanks, a dc current is supplied. The electrolyte isinitially supplied into the tank 56 from a chemical treatment storagetank, not depicted, via valved line 60. Solution is recirculated betweenthe chemical treatment tank and the chemical treatment storage tankduring the progress of the process with electrolyte flowing from thestorage tank into tank 56 via line 60 and returning to the chemicaltreatment tank by overflow gravity feed via line 62. The recirculationloop includes a heat exchanger, not depicted, where recirculatingsolution passes in indirect heat exchange with hot water to heat therecirculating liquid and control the temperature of solution in thestorage tank, for example to 105. A pair of wringer rolls 64 on eitherside of the strip at the exit end of tank 56 wipe electrolyte from thestrip and minimize the amount of the electrolyte leaving the tank 56 bydragout on the strip.

The tanks 52, 54 and 58 make up an interconnected washing system withwashing solution passing counterflow to the strip wherein the strip iswashed in a first washing step to recover an aqueous solution comprisinga relatively high concentration of hexavalent chromium in the form ofanions.

On startup, rinse solution, preferably demineralized water is introducedinto tank 52 via valved line 66. Also, rinse solution is initiallyintroduced into tank 54 through valved line 68 and into tank 58 througha valved line 70. Lines 68 and 70 are supplied through header 72 whichis supplied from a rinse water storage tank, not depicted.

During the process, demineralized water is added continuously into tank58 through spray nozzles 74 which are supplied by line 76 containingvalve 78. The spray nozzles 74 are on either side of the strip in tank58 on its up pass just prior to its passage over roll 58c. Flow throughthe spray nozzles 74 is perferably controlled to maintain a CrOconcentration in tank 54 equal to the CrO concentration in tank 56 bymanually setting the flow through the nozzles and controlling valve 78in response to periodic chemical tests carried out on tank 54 solution.These sprays aid in washing off residual electrolyte, that is aqueoushexavalent chromium containing solution, on the strip by dragout fromtank 56. Washing action is also provided by a bath 71 in the bottom oftank 58 through which the strip passes in its downpass and in its uppass through tank 58. The demineralized water introduced throughthe-sprays 74 and solution washed off of the strip by these sprays fallsby gravity into the lower portion of tank 58 and becomes part of thebath 71. Solution continuously overflows through an overflow outlet intank 58 providing a constant level in that tank.

The liquid leaving tank 58 through its overflow outlet enters aline 80and flows therethrough by gravity into a collection tank 82. The levelof liquid in, tank 82 is sensed by lever sensor 84. A level controller86 operates in response to the level sensed, and a pump 88 operating inresponse to a set point on the controller 86 operates to pump solutionfrom tank 82 via a line 90 into tank 54.

The tank 54 has an overflow outlet, and this acting in concert withaddition of solution through line 90 provides a substantially constantlevel bath in tank 54.

The strip passing through tank 54 is washed by the bath.

on its downpass and up pass through tank 54 whereby aqueous hexavalentchromium containing solution on the strip by dragout from the platingtanks and tank 20 is rinsed therefrom.

Solution passes out of the overflow outlet in tank 54 and passes via aline 92 into tank 52. A substantially constant level of rinse solutionis maintained in the lower portion of tank 52 as a result of an overflowoutlet in that tank. The bath of rinse solution in tank 52 provideswashing of the strip on its downpass from roll 52a to roll 52b and onits up pass from roll 52!; to roll 52c whereby aqueous hexavalentchromium containing solution on the strip by dragout from the platingtanks and tank 20 is washed therefrom.

Solution overflows through the outlet in tank 52, flows by gravitythrough a line 94 and then into main line 38 and through line 40 intoplater storage tank 32.

The concentration of chromic acid expressed as CrO in the baths in eachof the tanks 52, 54 and 58 is as follows: in the bath in tank 52, 85 to100 grams per liter; in the bath in tank 54, 30 to 40 grams Per liter,perferably 35; in the bath 71 in tank 58, 5 to gram per liter,preferably 10. These concentrations are controlled by control of valve78.

Following its exit from tank 58, the chrome plated strip still wettedwith some aqueous hexavalent chromium containing solution, for exampleone-tenth of the amount removed in the prior washing in tanks 52, 54 and58 continues along travel path 10 through a spray washer 96 depicted inFIG. 1B wherein the strip is washed in a second washing step to recoveran aqueous solution comprising cations (for example, iron and trivalentchromium ions) and a relatively low concentrationof hexavalent chromiumin the form of anions.

The spray washer comprises four open top compartments 98, 100, 102 and104 in series along its length with compartment 98 at its strip entranceend, followed by compartment 100, followed by compartment 102 andfinally followed by compartment 104 at its strip exit end. A wall 106separates compartments 98 and 100. A wall 108 separates compartments 102and 100. A wall 110 separates compartments 104 and 102. The

path and is horizontally oriented. There are five sets of wringer rollsthat guide the strip over the compartments in the horizontal path. Thesesets of wringer rolls are respectively denoted 98a, 100a, 102a, 104a andl04b with set 98a being at the strip entrance end of the washer overcompartment 98, set 100a being over compartment 100 near wall 106, set102a being over compartment 102 near wall 108, set 104a being overcompartment 104 near wall 110 and l04b being near the strip exit end ofthe washer 96.

Over each of the compartments 98, 100, 102, 104 there are two spraynozzles, one below the strip and one above the strip so that the stripis sprayed from above and below. The spray nozzles above tank 98 aredenoted 98c; those above compartment 100 are denoted l00c; those abovecompartment 102 are denoted 102C and those above compartment 104 aredenoted 104C. Spray nozzles 1040 are supplied with demineralized waterby valved line 112. The water from the spray nozzles 104C washesresidual solution containing hexaof compartment 102 through a valvedline 114 by a pump 116 to the spray nozzles 1020. The solution from thenozzles 102 washes residual solution containing hexavalent chromium fromthe strip and resultant solution falls into compartment 102. Thesolution in compartment 102 accumulates to the level of the top of wall108 and then continuously cascades over this wall into compartment 100.Solution from the bottom of compartment flows through a valved line 118and is pumped by a pump 120 to spray nozzles 100;. Solution from thespray nozzles 100s washes residual solution containing hexavalentchromium from the strip and the resultant solution falls intocompartment 100. Solution accumulates in compartment 100 to fill thatcompartment and thereafter cascades over wall 106 into compartment 98.Solution flows from the bottom of compartment 98 through a valved line122 and is pumped by a pump 124 to spray nozzles 98c. Solution from thespray nozzles 98c washes residual solution containing hexavalentchromium from the strip and the resultant liquid falls into compartment98. Solution overflows from compartment 98 into line 126.

. The spray washer 96 constitutes a counterflow spray washing systemwhereby solution on the strip containing hexavalent chromium is washedtherefrom and a solution comprising cations (for example, iron andtrivalent chromium ions) and a relatively low concentration ofhexavalent chromium, that is, for example, from 0.1 to 0.3 grams perliter of chromic acid expressed as CrO overflows into line 126. When thestrip leaves washer 96, it contains only trace or no amounts of chromicacid wetting it.

The strip exiting from the spray washer is dried and coiled.

Merging with line 126 is a line 128 containing a metering pump 130. Theupstream end of line 128 communicates with a chrome waste storage tank,not shown in the drawings. Various of the tanks can be emptied into thechrome waste storage tank, for example on shut down, to capture chromeladen water which can be metered by pump 130 into line 126 for chromerecovery.

Solution from line 126 enters a collection tank 132 having an exit line134.

Valved line 133 is provided communicating with tank 132 for the passagethereto of chrome laden water from sources other than compartment 98 andline 128; for example, chrome laden water from washing strip exitingfrom a chemical treatment step in an electrotinplating line can bepassed into tank 132 through line 133.

The valved lines 128 and 133 constitute means for admixing aqueoussolution recovered from the second washing step with hexavalent chromiumcontaining solution from other sources to provide in tank 132 anadmixture which is an aqueous solution comprising cations and arelatively low concentration of hexavalent chromium in the formofanions.

Solution is pumped from tank 132 by a pump 136 in line 134 in responseto a level control 138 operating a valve 140 in line 134.,The pump 136runs continuously and the level control operates to close the valve moreif the level in the tank drops. This setup operates to providecontinuous flow through line 134.

The solution from line 134 flows through heat exchanger 142 where it iscooled against the countercurrent flow of cold water as denoted byarrows 142a and 142b. Cooling is to a temperature suitable for treatmentof the solution in the ion exchange system which is described below.

Solution leaves the heat exchanger 142 via a line 144 and passes througha filter 146 which removes particles of solid material. The filteredsolution flows through line 148 into an ion exchange system which isdescribed below.

The ion exchange system comprises three sets of ion exchangers, each setconsisting first of a cation exchanger defining a first cation exchangezone and then of an anion exchanger defining an anion exchange zone. Thecation exchangers are denoted 150a, 1501: and 1500. The anion exchangerassociated with cation exchanger 150a is denoted 152a; the anionexchanger associated with cation exchanger 15012 is denoted 152b," andthe anion exchanger associated with cation exchanger 1500 is denoted1520. Valved lines 154a, 154b and 1540 respectively communicate betweenline 148 and cation exchangers 150a, 150b, and 1500. Line 156acommunicates between cation exchanger 150a and anion exchanger 152a;line 1561) communicates between cation exchanger 15011 and anionexchanger 15211; and line 1560 communicates between cation exchanger1500 and anion exchanger 1520. The lines 1560, 156b and 1560 eachcontain valves. Three sets of the ion exchangers are provided so thattwo sets can be on line while the exchangers of the other set are beingregenerated.

Resins for use in the cation exchangers 150a, 150b and 1500 are of thestrong acid type and are in the hydrogen form. They contain sulfonicacid functional groups and are prepared, for example, by the nuclearsulfonation of styrenedivinylbenzene. These resins are not as highlyoxidative resistant as the resins utilized in the cation exchanger fortreating regeneration effluent which is described later. Suitable cationexchange resins are, for example, Amberlite IR-l20 or Amberlite IR-l20Plus manufactured by Rohm and Haas Company.

The ion exchange resin for use in the anion exchangers is a strong baseanion exchange resin of the quaternary ammonium type. It is utilized inthe hydroxide form. A suitable resin is Amberlite IRA-900 which issupplied in the chloride form and is converted to the hydroxide form foruse; it is available from Rohm and Haas Company.

Solution flows from line 148 into whichever two of the cation exchangersa, 15% and 1500 are on stream. The cation exchangers take out cations,including trivalent chromium and iron ions and replace them withhydrogen ions. Effluent from a cation exchanger is aqueous solutioncontaining hexavalent chromium in the form of anions, other anions andhydrogen ions and flows into an associated anion exchanger 152a, 152b or1520. The anion exchangers take out hexavalent chromium in the form ofanions and other anions and replace these with hydroxyl ions. Effluentfrom the anion exchangers is demineralized water. Valved exit lines forflow of effluent from anion exchangers 152a, 152b and 1520 arerespectively denoted 158a, 158k and 1580.

The lines 158a, 158b and 1580 each communicate with a main line 160whereby demineralized water effluent fiows into a demineralized waterstorage tank 162. Included in the line 160 is a device 164 fordeaerating the demineralized water. The storage tank 162 has a valvedexit line 166 for flow of water to lines 112 (see FIG. 1B) and 76 (seeFIG. 1A). A pump 168 is provided in line 166 to pump the water throughline 166.

When a set of ion exchange units is to be regenerated, it is isolatedfrom flow from line 148 and from line 160 by closing the appropriatevalves, and another set of ion exchange units is brought intocommunication with line 148 and line 160.

Regeneration of the cation exchange unit in a set of units is carriedout, for example as suggested by the manufacturer of the particularresin utilized, and the particular method of regeneration forms no partof the present invention. Regeneration of a cation exchange unit can becarried out for example, by backwashing, introducting 10% sulfuric acidas a regenerating agent and rinsing.

Regenerating of an anion exchange unit involves a regeneration cyclecomprising backwashing, removal of residual backwash water, introductionof regenerating agent, removal of resulting solution, introduction ofrinse water and removal of resulting solution, introduction of water,and recirculation. For simplification purposes some valves associatedwith these steps are not depicted. For further simplification, the stateof each valve in the system at each point in time during the cycle willnot be described; the conduits described as functioning at a particularpoint in time are open or have valves positioned as otherwise describedand the other conduits in the regeneration system at that point in timeare closed unless otherwise stated. The regeneration is carried out toproduce an aqueous effluent comprising cations (from the regeneratingagent) and a concentration of hexavalent chromium in the form of anionssubstantially higher than the relatively low concentration leavingcompartment 98 or in tank 132. Re-

generation of an anion exchange unit is described in detail below.

Backwashing of the anion exchange column is carried out to fluff up theresin and remove extraneous solids and resin fines from the resin bed topermit good contact between the resin and regenerating agent so thatregeneration is carried out efficiently. Backwash water having cationsremoved therefrom, for example by passage through the cation exchangerassociated with the anion exchanger being regenerated, is introducedinto the bottom of the anion exchange column (the flows other thanbackwash flows through the various ion exchange columns herein are fromthe top as is conventional; however, the direction of flow constitutesno part of the present invention). Piping from the introduction ofbackwash water is represented by valved line 169a, and piping for outletof backwash water is represented by valved line 169b, both depicted asassociated with column 1526. Corresponding piping is provided forcolumns 152a and 15212 but such is not depicted. When backwashing hasbeen completed, the valve in the appropriate backwash water introductionline (for example 169a) is closed. Residual backwash water is removedfrom the anion exchanger utilizing a pressurized air purge. An airsupply line 170 supplies pressurized air for this purpose withcommunication between line 170 and exchangers 152a, 152b and 1526 beingprovided respectively by valved lines 172a, l72b and 1720. Solutionleaves the column during backwashing and the subsequent purging viabackwash outlet line (for example, line 16%) and is routed to wastedisposal.

After backwash water has been so purged, pressurized air flow is stoppedby closing the valve in the appropriate line 172a, 172b or 172C and thevalve in the appropriate backwash outlet line, for example line 169b, isclosed. The anion exchanger being regenerated is then vented to theatmosphere by means of a valve not depicted. Then, regenerating agent isflowed into the anion exchanger. For this purpose, a regenerating agentsupply line 174 is provided, and it communicates respectively with anionexchangers 152a, 152b and 1526 by means of valved lines 176a, 176b and176C. The regenerating agent comprises alkali metal hydroxide, forexample, potassium or sodium hydroxide or mixtures thereof. Preferably,the regenerating agent comprises sodium hydroxide utilized in aqueoussolution at a concentration of by volume. The alkali metal hydroxidereacts with the resin to provide resin in the hydroxide form and releaseanions containing hexavalent chromium and produce alkali metal saltscontaining hexavalent chromium in a negative radical, that is alkalimetal chromates and dichromates. The amount of alkali metal hydroxideintroduced is based on calculations of how much chromium entered andtherefore is in a particular column with a sufficient amount being addedso that when solution is removed from the column as described hereafteras the effluent of regeneration as defined hereafter, the effluent ofregeneration will contain from about 90 to about 95% of the hexavalentchromium in the column just prior to initiation of regeneration. Whenthe appropriate amount of regenerating agent has been introduced, flowof such into the column is stopped, and liquid is forced from the columnutilizing a pressurized air purge. The pressurized air for the air purgeis supplied through line 170. When the air purge has forced the liquidfrom the column that readily is removed thereby, air introduction iscontinued and rinse water is simultaneously routed into the column for apredetermined time and is forced therefrom by the pressurized air. Rinsewater flow is provided through piping described below. The streamsleaving the column both as a result of purging of liquid in the columnfrom addition of regenerating agent and as a result of purging liquidfrom the rinse water introduction make up the effluent of regeneration.These streams leave the columns 1520, 152b and 1520 respectively throughexit lines 178a, 178b and 1786. Thus, alkali metal hydroxide isintroduced to provide an aqueous effluent from the column containingcations and from aboout 90 to about 95% of the hexavalent chromiumpresent in a column just prior to initiation of regeneration and toleave in the anion exchange column the remainder of the cations andhexavalent chromium. Preferably, the amount of hexavalent chromiunremoved from the column in the effluent is about 95% of that presentjust prior to initiation of regeneration.

The exit lines 178a, 178b and 178C communicate respectively withthree'way valves 180a, 18% and 1800. During forcing of effluent ofregeneration from a column, the appropirate three-way valve ispositioned so 4 as to route liquid into a respective communicating line182a, 182b or 1826. The lines 182a, 18211 and 1820 communicate with amain line 184.

After the effluent of regeneration has left the column being regeneratedand has been routed to line 184, the pressurized air supply is stoppedby closing of the valve in the appropriate line 172a, l72b or 172e, butwater introduction is continued to fill the column and simultaneouslywith the shutting off of the air supply the appropriate three-way valvea, 1801) or 1800 is repositioned so as to connect the respective line178a, l78b or 1786' into a recirculation loop described below. Pipingfor this water introduction as well as the rinse water introduction toprovide part of the effluent of regeneration comprises a main water line186 which in turn communicates with valved branch lines 188a, 188!) and1886 which respectively communicate with cation exchanger entrance linesa, 19% and 190C with 188a and 190a being associated with exchanger 150a,188b and 19% being associated with exchanger 15012 and 188C and 1906being associated with exchanger 1500. The piping for the water additionsystem also includes a pipe 192a connecting exchangers 150a and 152a, apipe 192b connecting exchangers 150i) and 152b, a pipe 192C connectingexchangers 150C and 1526, the pipes 192a, 19212 and 1926' each containvalves which are not depicted. Thus, in introducing water into anionexchanger 152a, thewater from main line 186 enters valved line 188a andthen line 190a, then passes through cation exchanger 150a, then throughline 192a and into anion exchanger 152a to fill the same. The exchangers15211 and 1520 are filled with water in corresponding fashion when theyare in a regeneration cycle. The passage of the water through the cationexchanger before entry into the anion exchanger provides the advantageof removing cations therefrom including divalent cations such as calciumand magnesium ions so that service water can be utilized without dangerof hardness in the water clogging the exchanger system. In other words,the water can be drawn from a source of water containing divalentcations (for example, calcium and magnesium ions) since these cationsare removed prior to entry of the water into the anion exchanger.

Once the anion exchange column has been filled with water, wateraddition is stopped by closing the valve in the appropriate line 188a,188b, 1880, the appropriate vent valve is closed, and recirculationbetween the anion exchange column and its associated cation exchangecolumn is started. The recirculation loop including ion exchange columns150a and 152a consists of line 178a, a line 194a containing a pump 196a,valve 180a joining lines 178a, 182a and 194a and positioned to provideflow between lines 178a and 194a, line 190a communicating at itsupstream end with the downstream end of line 194a, and line 192a. Therecirculation loop including ion exchange columns 150b and l52b consistsof line 178b, a line 194b containing a pump 196b, valve 180b joininglines 178b, 182k and 194b and positioned to provide flow between lines178b and 194b, line 190b communicating at its upstream end with thedownstream end of line 194b, and line 192b. The recirculation loopincluding ion exchange columns 150c and 1520 consists of line 1780, aline 194c containing a pump 196e, valve 1800 joining lines 178c, 182cand 194C and positioned to provide flow between lines 1780 and 194e,line 1900 communicating at its upstream end with the downstream end ofline 1940, and line 1920. To carry out recirculation, the appropriatepump 196a, 196b or 1966 is operated. Recirculation is carried outbetween the anion exchanger and the cation exchanger in a loop untilanions including the hexavalent chromiun containing anions in the anionexchanger being treated react with the anion exchanger resin in thatexchanger and cations react with the resin in the cation exchanger.Conductivity sensors 198a, 198b and 198c are provided respectivelysensing in lines 178a, 178b and 178C to indicate when reaction has beencompleted.

After reaction has been completed, recirculation is stopped by stoppingthe appropriate pump 196a, 196b or 1960.

Preferably, the regeneration cycle including backwashing, air purging,introduction of regenerating agent, removal of resulting solution,introduction of rinse water and removal of resulting solution,introduction of water and recirculation is carried out automatically sothat the various valves including the valves in lines 188a, 188b, 188C,158a, 158b and 1586, 169a, 16% and the valves 180a, 18% and 1806 areoperated automatically in accordance with a predetermined schedule.

The liquid passing through line 184 comprises an aqueous solutioncontaining cations (alkali metal ions from the regenerating agent forthe anion exchange zone) and a concentration of hexavalent chromium inthe form of anions substantially higher than the relatively lowconcentration leaving compartment 98 or in tank 132. It flows into aholding tank 200. Intermittently liquid is pumped from tank 200 by pump202 through a valved line 204 through a cation exchange column 206referred to hereafter as cation exchanger 206, exchanger 206 and column206 (see FIG. 1A). Effluent from the cation exchanger passes into platersolution storage tank 32 via a line 208. When the liquid passes throughthe cation exchanger 206, cations in the liquid react with a resin inthe exchanger and are exchanged for hydrogen ions, and the effluent fromthe cation exchanger is an aqueous solution of chromic acid containing,for example to 50 grams per liter of chromic acid expressed as CrO thatis an acidic aqueous solution comprising a concentration of hexavalentchromium in the form of anions substantially higher than theconcentration of such in the stream leaving compartment 98 through line126 or in tank 132. Me ters 210a and 210b (pH meters) are provided oneither side of exchanger 206 for the easy determination of whether theresin bed in exchanger 206 is depleted, that is whether the resin in theexchanger is spent and needs to be regenerated.

The ion exchange resin utilized in cation exchanger 206 is of strongacid type and is used in the hydrogen form. It contains sulfonic acidfunctional groups in a polymer matrix and is prepared, for example bythe nuclear sulfonation of styrene-divinylbenzene. The resin utilized ishighly resistant to oxidation because the concentration of chromic acidto which the column is exposed is such as to be highly oxidative. Theresin contains a relatively high level of cross-linking to provide suchoxidation resistance. A suitable resin is sold under the tradenameAmberlite 200 by Rohm and Haas; this resin is sold in the sodium formand is converted to the hydrogen form for use.

The solution leaving compartment 98 through line 126 contains, forexample, 0.1 to 0.3 grams per liter of chromic acid expressed as CrO andthe solution leaving the exchange system through line 208 contains forexample 10 to 50 grams per liter of chromic acid expressed as CrO Thus,the ion exchanger system serves to concentrate the chromic acid solutionso that is can be treated as described hereafter so as to be suitablefor reuse in the plater tanks.

An evaporation system is provided to control the volume of liquid in thesystem and to remove water from the system inasmuch as liquid from line94 comprises chromic acid at a concentration of, for example, to gramsper liter of chromic acid expressed as CrO and the liquid from line 208has a concentration of chromic acid expressed as CrO of, for example, 10to 50 grams per liter. In other words, the water added via line 76 andthe excess water added into the system through line 174 has to beremoved to concentrate solution to plating strength. The evaporationsystem comprises an evaporator 211 defining an evaporation zone wheresolution is heated against steam passing in indirect heat exchangerelation as indicated by lines 212a, and 212b. The evaporator has aconcentrated liquid exit line 214 anda vapor exit line 216. Alsocommunicating with thee vaporator is a liquid feed line 218 containing apump 220 and a valve 222 and communicating at its upstream end with thebottom of plating tank 32. In utilizing the evaporator, solution fromplater storage tank 32 is passed from the plater storage tank to theevaporator and concentrated solution is passed from the evaporator tothe plater storage tank. More particularly, the liquid is pumped by pump220 from the lower portion of plater storage tank 32 through line 218into the evaporator where is is heated indirectly against steam passingas indicated by lines 212a and 21217 and subjected to vacuum wherebywater is flashed off leaving through line 216 (such water is condensedand disposed to waste) and concentrated liquid leaves the evaporator vialine 214 and is fed back into plater storage tank 32.

An inventive concept herein concerns the addition of regenerating agentto provide an effluentcontaining from about 90 to about 95% of thehexavalent chromium present in the anion exchange resin just prior toinitiation of regeneration and then recirculating the remaining tohexavalent chromium and reacting it with the anion exchange resin toestablish a continuing level of hexavalent chromium in the resin. Thisprocedure maximizes the concentration of hexavalent chromium in thesolution flowing into tank 200 from line 184. Once the 5 to 10% level ofhexavalent chromium is established in the resin, additional chromiumneed not be utilized to establish this level during the life of theresin; therefore, the establishment of this level is provided withminimum usage of'hexavalent chro EXAMPLE The system of FIGS. 1A and 1Bis utilized. The system is generally operated as described above. Thespecities are presented below.

Steel strip (36 inch width) having been preliminarly treated inelectrolytic cleaning and pickling steps is passed through the system.Chrome plating is applied in tanks 12, l4, l6 and 18. Objectionableoxide coating is removed in tank 20. The plated strip is washed in tanks52 and 54. In tank 56, hexavalent chrome oxide coating is applied. Intank 58 the strip is washed. Finally the strip is further washed inspray washer 96. The strip exiting from washer 96 is ready for drying.The line speed is 1,550 feet per minute.

Each of the tanks l2, l4, 16, 18 and 20 contains 1,700 gallons of platersolution. The plater solution is an aqueous solution comprising 150grams per liter of chromic acid expressed as CrO -1 gram per liter ofsulfuric acid expressed as S0 and 3 grams per liter of metalsilicofluorides expressed as SiF 100 gallons per minute of platersolution enters each tank and 100 gallons per minute of plater solutionleaves each tank. Of the 100 gallons per minute entering tank 12, 2.gallons per minute enters through prewet sprays 19. The temperature ofthe plater solution is 1 Tank 52 contains 800 gallons of aqueoussolution containing about 90 grams per liter of chromic acid expressedas CrO Solution enters and leaves tank 52 at the average rate of about1.5 gallons per minute.

Tank 54 contains 1,700 gallons of aqueous solution containing about 35grams per liter of chromic acid expressed as CrO Solution enters andleaves tank 54 at the average rate of about 1.5 gallons per minute.

Tank 56 contains 1,700 gallons of aqueous solution containing chromicacid at a concentration of about 35 grams per liter expressed as CrO0.15 grams per liter of sulfuric acid expressed as $0., and 0.6 gramsper liter of metal silicofluorides expressed as SiF Solution passes inand out of tank 56 at a rate of approximately 150 gallons per minute.The temperature of solution in tank 56 is 120.

Tank 58 contains 800 gallons of aqueous solution. This solution has aconcentration of chromic acid expressed as CrO of 10 grams per liter.1.5 gallons per minute of solution continuously pass in and out of thistank.

In washer 96 20 gallons per minute of demineralized water enters throughsprays 104c; 20 gallons per minute overflows from compartment 104 tocompartment 102; 20 gallons per minute is sprayed by sprays 1020; 20gallons overflows from compartment 102 to compartment 100; 20 gallonsper minute is sprayed through sprays 100C, 20 gallons per minuteoverflows from compartment 100 to compartment 98; 20 gallons per minuteis sprayed through sprays 98c; and 20 gallons per minute leaves throughline 126. the water entering through sprays 104C is at a temperature of180. The use of water at this temperature aids in strip washing anddrying.

The solution leaving through line 126 has a concentration of chromicacid expressed as CrO of 0.2 grams per liter. The concentration ofchromic acid expressed as CrO in compartment 104 is in low parts permillion. The strip leaving washer 96 contains trace or no amounts ofchromic acid.

. Tank 132 is a 6,000 gallon tank and controller 138 operates valve 140to maintain a continuous stream of liquid through pipe 134. This streamof liquid is an aqueous solution having a concentration of hexavalentchromium in the form of anions expressed as CrO of 0.2 grams per liter.

The heat exchanger 142 operates to cool the solution from pipe 134 to100.

The solution having been so cooled is routed to two of the three cationexchangers 150a, 150b and 1506. These exchangers and each of the otherion exchangers in the system are 10 feet high and 54 inches in diameterand of the conventional type where the ion exchange resin is maintainedupon a screen which is positioned in the bottom of the exchanger, theexchanger is vertically oriented and the solution to be treated entersthe top and leaves the bottom. The ion exchange resin utilized in theexchangers 150a, l50b and 1506 is Amberlite IR- Plus, and it is utilizedin the hydrogen form. The effluent from the two cation exchangers (of a,l50b and 1500) that are on stream is passed to the two anion exchangerswhich communicate with such cation exchangers. The anion exchangers152a, 152b and 152C contain as an ion exchange resin Amberlite IRA-900.This resin is supplied in the chloride form and is converted to thehydroxide form for use. The effluent from the anion exchangers isdemineralized water which is routed to storage tank 162.

. Every eight hours one set of cation and anion exchangers is taken outof the line and a fresh set of cation and anion exchangers is put in theline. For example, if cation exchangers 150a and 150b and anionexchangers 152a and 152b are being utilized and the resin in exchangers150a and 152a is the most depleted, exchangers 150a and 152a are takenout of the line and exchangers 150C and 152c are put in the line.

The cation exchanger taken out of the line is regenerated prior toregenerating the associated anion exchanger. The regeneration comprisesbackwashing, then treating with a regenerating agent consisting of 10%aqueous sulfuric acid and then rinsing. Flow rates and times are inaccordance with the resin manufacturers recommendations. i

The anion exchanger which is taken out of the line is regenerated firstby backwashing for 10 minutes utilizing service water having cationsremoved therefrom introduced through the appropriate backwash waterintroduction line. When the ten-minute time period has ended, the valvein the backwash water line is closed automatically; then the appropriatevalve in the air purge line (that is, the valve in the appropriatebranch line 172a, l72b or 172a) is automatically opened and the residualbackwash water is forced from the anion exchanger through theappropriate backwash water outlet line. After the valve for air purginghas been open for 10 minutes, it automatically closes and the anionexchanger is automatically vented to the atmosphere.

Then the valve in the appropriate regenerating agent introduction line176a, 1761: or 1760 automatically opens. The regenerating agent utilizedis aqueous sodium hydroxide solution containing 10% sodium hydroxide byvolume. Four hundred gallons of regenerating agent is introduced over a40 minute time period. At this point, the valve in the regeneratingagent introduction line automatically closes, and the valve in theappropriate line 172a, 172b or 1726 automatically opens and theappropriate three-way valve 180a, 18017 or 1801' is automaticallypositioned to route liquid to tank 200. After 10 minutes, the air hasforced substantially all of the liquid from the column beingregenerated. At this point the valves in the appropriate lines 188a,188b, 1880, 1920, 192b, 1926 automatically open whereby water passesthrough a cation exchanger and then into the anion exchanger which isbeing regenerated. The water is introduced at the rate of 60 gallons perminute. After 1 minute, the appropriate valve is closed to shut off theair. The 60 gallons of water introduced before the air is shut offserves to rinse out residual solution heavily laden with chrome, andcarry it to tank 200. The 460 gallons routed to tank 200 contain 95% ofthe hexavalent chromium which was in the resin in the column just priorto initiation of regeneration.

When the air is shut off, the appropriate three-way valve 180a, 1801; or1800 is automatically repositioned to recirculation position (that is,to provide communication, for example between lines 178a and 194a) andwater introduction is continued to fill the column. Then waterintroduction is automatically stopped by means of a conductivity switch,the vent valve closes and the appropriate pump 196a, 19612 or 1966 isautomatically started and recirculation between the anion exchangerbeing regenerated and its associated cation exchanger is carried out.This recirculation is continued until the conductivity cell 198a, 198bor 1980 registers 25 micromhos thereby indicating reaction of the sodiumchromates and dichromates with the resins to establish a .level chromateand dichromate in the anion exchange resm.

Except during regeneration of column 206, solution is pumped by pump 202from tank 200 through exchanger 206. The exchange resin in thisexchanger is Amerlite 200 which has been converted to the hydrogen form;it is much more highly oxidative resistant than the cation exchangeresin Amberlite IR-l Plus utilized in the cation exchangers 150a, 1501;and 150C. Passage of the solution through the cation exchange resinremoves cations and the effluent from the cation exchanger 206 isaqueous chromic acid solution containing grams of chromic acid per literexpressed as CrO The differential pH as indicated by meters 210a and210b indicates resin bed depletion, and a predetermined higher pH beingregistered by meter 210a indicates that the resin bed is ready to beregenerated. Re-

generation is carried out utilizing backwashing, introduction ofregenerating agent and rinsing in accordance with the manufacturersdirections for the particular resin being utilized. The regeneratingagent utilized is 10% by volume aqueous sulfuric acid.

Tank 32 contains about 6,000 gallons when the line tanks are full.

Recirculation is carried out between tank 32 and evaporator 211 withgallons per minute being pumped by pump 200 whereby the concentration ofsolution in tank 32 is maintained at 150 grams per liter of chromic acidexpressed as CrO Steam is introduced through line 212a to supply heat. Avacuum of 26 inches of Hg is utilized in evaporator 21 1; this vacuum isprovided by an eductor on line 216.

The recirculation through lines 44 and 46 between heat exchanger 42 andtank 32 is at the rate of 500 gallons per minute and cooling water ispassed through exchanger 42 via lines 50a and 50b countercurrent to theflow of plater solution to remove heat from the system and maintain thetemperature of the plater solution at 1 15F.

The use of different cation exchangers for the cation exchange upstreamof the anion exchange and for the cation exchange downstream of theanion exchange allows the use of different cation exchange resins sothat a highly oxidative resistant resin can be utilized in exchanger 206where such resin is desirable because of the relatively highconcentration of hexavalent chromium in the form of anions while a resinwhich is not highly oxidative resistant but which is less expensive canbe utilized in exchangers 150a, l50b and 1500 where the concentration ofhexavalent chromium in the form of anions is lower and less oxidationresistance is needed. Thus, the use of different cation exchangers anddifferent resins significantly contributes to the economy of the system.The removal of only of the hexavalent chromium from the anion exchangers152a, 1521; and 1526 during regeneration maximizes the concentration ofchromic acid in the effluent of regeneration thereby significantlycontributing further to the economy of the system by reducing the loadon the evaporation system. Finally, the recirculation between the platertanks and tank 32 in combination with adding the streams from lines 94and 208 into tank 32 and treating solution from tank 32 in theevaporator provides significantly more accurate plater solution volumeand concentration control than if streams 94 and 208 were passeddirectly to an evaporator and has the further advantage of allowingcontinuous evaporator operation on a relatively large volume of liquidcompared to the volume of thestreams from 94 and 208 thereby permittingcontinuous evaporator operation without danger of exceeding the flashpoint of the solution being treated.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. Therefore, in viewof the variations that are readily understood to come within the limitsof the invention, such limits are defined by the scope of the appendedclaims.

What is claimed is:

1. Method both forrecovering chromium from strong base anion exchangeresin which has been used in the hydroxide form to remove hexavalentchromium in the form of anions from a solution and for regenerating theresin for reuse for removing hexavalent chromium in the form of anionsfrom a solution; comprising the steps of a. introducing water into acolumn containing the resin thereby backwashing the resin to removeextraneous solids and resin fines,

b. removing residual backwash water,

0. passing aqueous alkali metal hydroxide into the column containing theresin and providing effluent from the column containing from about 90 toabout 95% of the hexavalent chromium present in the column while leavingin the column alkali metal cations and the remainder of the hexavalentchromium in the form of anions to thereby optimize the concentration ofhexavalent chromium in the effluent,

d. introducing water into the column and recirculating resulting liquidbetween that column and a column containing cation exchange resin of thestrong acid type in the hydrogen form to react hexavalent chromium inthe form of anions with the anion exchange resin and to react the alkalimetal cations with the cation exchange resin, to establish a continuingto level of hexavalent chromium in the anion exchange resin therebyconserving chromium.

2. Method as recited in claim 1, in which the alkali metal hydroxide issodium hydroxide.

3. Method as recited in claim 1, in which in step (b) residual backwashwater is removed utilizing pressurized air 4. Method as recited in claim3 in which liquid resulting in the column containing the anion exchangeresin from passing aqueous alkali metal hydroxide thereinto is removedfrom the column utilizing pressurized air.

5. Method as recited in claim 4 in which subsequent to the removal ofliquid resulting in the column containing the anion exchange resin frompassing aqueous alkali metal hydroxide thereinto and prior to step (d),rinse water is introduced into that column and resulting solution isremoved utilizing pressurized air to provide a portion of the effluentof step (c).

6. Method as recited in claim 5 in which the rinse water and pressurizedair are introduced into the column containing the anion exchange resinsimultaneously.

7. Method as recited in claim 1 in which the'water introduced in step(d) is passed through a column containing cation exchange resin of thestrong acid type in the hydrogen form prior to such introduction 8.Method as recited in claim 1 wherein in step (c) the effluent containsabout of the hexavalent chromium.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 885Dated 20 19 75 lnventor( ert B. Smlth It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 19 "1000" should read 1005 Column 2 line 17 "50" shouldread so Column 16 line 9 "200" should read 220 Signed and Scaled thistwenty-third a) or September 1975 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN :"H X lfi (nmnlissium'r uflarcnrs andTrurlcmurkx USCOMM-DC 60376-5 69 U S GQVKRNME'JY PRXNYING OFFICE a 9 9FORM PO-105OUO-69)

1. METHOD BOTH FOR RECOVERING CHROMIUM FROM STRONG BASE ANION EXCHANGERESIN WHICH HAS BEEN USED IN THE HYDROXIDE FORM TO REMOVE THE HEXAVALENTCHROMIUM IN THE FORM OF ANIONS FROM A SOLUTION AND FOR REGENERATING THERESING FOR REUSE FOR REMOVING HEXAVALENT CHROMIUM IN THE FORM OF ANIONSFROM A SOLUTION; COMPRISING THE STEPS OF A. INTRODUCING WATER INTO ACOLUMN CONTAINING THE RESIN THEREBY BACKWASHING THE RESIN TO REMOVEEXTRANEOUS SOLIDS AND RESIN FINES, B. REMOVING RESIDUAL BACKWASH WATER,C. PASSING AQUEOUS ALKALI METAL HYDROXIDE INTO COLUMN CONTAINING THERESIN AND PROVIDING EFFLUENT FROM THE COLUMN CONTAINING FROM ABOUT 90 TOABOUT 95% OF THE HEXAVALENT CHROMIUM PRESENT IN THE COLUMN WHILE LEAVINGIN THE COLUMN ALKALI METAL CATIONS AND THE REMAINDER OF THE HEXAVALENTCHROMIUM IN THE FORM OF ANIONS TO THEREBY OPTIMIZE THE CONCENTRATION OFHEXAVALENT CHROMIUM IN THE EFFLUENT, D. INTRODUCING WATER INTO THECOLUMN AND RECIRCULATING RESULTING LIQUID BETWEEN THAT COLUMN AND ACOLUMN CONTAINING CATION EXCHANGE RESIN OF THE STRONG ACID TYPE IN THEHYDROGEN FORM TO REACT HEXAVALENT CHROMIUM IN THE FORM OF ANIONS WITHTHE ANION EXCHANGE RESIN AND TO REACT THE ALKALI METAL CATIONS WITH THECATION EXCHANGE RESIN, TO ESTABLISH A CONTINUING 5 TO 10% LEVEL OFHEXAVALENT CHROMIUM IN THE ANION EXCHANGE RESIN THEREBY CONSERVINGCHROMIUM.
 2. Method as recited in claim 1, in which the alkali metalhydroxide is sodium hydroxide.
 3. Method as recited in claim 1, in whichin step (b) residual backwash water is removed utilizing pressurizedair.
 4. Method as recited in claim 3 in which liquid resulting in thecolumn containing the anion exchange resin from passing aqueous alkalimetal hydroxide thereinto is removed from the column utilizingpressurized air.
 5. Method as recited in claim 4 in which subsequent tothe removal of liquid resulting in the column containing the anionexchange resin from passing aqueous alkali metal hydroxide thereinto andprior to step (d), rinse water is introduced into that column andresulting solution is removed utilizing pressurized air to provide aportion of the effluent of step (c).
 6. Method as recited in claim 5 inwhich the rinse water and pressurized air are introduced into the columncontaining the anion exchange resin simultaneously.
 7. Method as recitedin claim 1 in which the water introduced in step (d) is passed through acolumn containing cation exchange resin of the strong acid type in thehydrogen form prior to such introduction
 8. Method as recited in claim 1wherein in step (c) the effluent contains about 95% of the hexavalentchromium.